The prior technology is classified into two groups. The first group uses a table which stores addresses of the pels of the original image. The addresses represent the pels of an original image which are selectively picked up to form a scaled down image or a reduced image. In an enlargement of the original image, the addresses are used which represent the pels of the original image which are used one time or plural times in accordance with a ratio of enlargement. An article "System for converting resolution," IBM Technical Disclosure Bulletin, Vol. 31, No. 6, November 1988, pp. 324-327 shows the example of the table for reducing the number of pels of the original image. The problem of this technology is that various tables must be prepared to adapt to the various scale ratios.
The second group uses an algorithm for scaling the original image without requiring the tables of the technology of the first group. A typical algorithm is Bresenham's algorithm which was described in the IBM Systems Journal, Vol. 4, No. 1, 1965. The Bresenham's algorithm is known as a line generator which is used for scaling up or down the image. The problem in the use of the Bresenham's line generator is that two Bresenham's line generator are required to perform both the scaling up and scaling down operations, and additional program steps for discriminating which operation is selected are required. The use of the two line generators increases the cost and decreases the performance of the operations.
Another typical algorithm is known as a digital differential analyzer (DDA). It is assumed that 50 pels are reduced or scaled down to 30 pels. First, a calculation 50/30=1.6666 . . . is performed. It is assumed that the value 1.666 is used. The value 1.666 is stored in a register and the stored value is updated by repeatedly adding the value 1.666 to the stored value. The updated values are 1.666, 3.332, 4.998, 6.664, . . . . The integral part of the value is used as an address of the pel of the original image to be picked up to form the scaled down image. In this example, first, third, fourth, sixth pels, etc. of the original image are picked up to form the scaled down image. The problem of the DDA is that the DDA inherently generates an error due to the division, and the DDA requires a long calculation time, so that the performance of the DDA is quite low in comparison with the present invention.